Current Gaps in Understanding the Molecular Basis of FXTAS

Non-canonical DNA/RNA structures associated with the pathogenesis of Fragile X-associated tremor/ataxia syndrome and Fragile X syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X syndrome (FXS) are primary examples of fragile X-related disorders (FXDs) caused by abnormal expansion of CGG repeats above a certain threshold in the 5′-untranslated region of the fragile X mental retardation (FMR1) gene. Both diseases have distinct clinical manifestations and molecular pathogenesis. FXTAS is a late-adult-onset neurodegenerative disorder caused by a premutation (PM) allele (CGG expansion of 55–200 repeats), resulting in FMR1 gene hyperexpression. On the other hand, FXS is a neurodevelopmental disorder that results from a full mutation (FM) allele (CGG expansions of ≥200 repeats) leading to heterochromatization and transcriptional silencing of the FMR1 gene. The main challenge is to determine how CGG repeat expansion affects the fundamentally distinct nature of FMR1 expression in FM and PM ranges. Abnormal CGG repeat expansions form a variety of non-canonical DNA and RNA structures that can disrupt various cellular processes and cause distinct effects in PM and FM alleles. Here, we review these structures and how they are related to underlying mutations and disease pathology in FXS and FXTAS. Finally, as new CGG expansions within the genome have been identified, it will be interesting to determine their implications in disease pathology and treatment.

Fragile X Premutation rCGG Repeats Impair Synaptic Growth and Synaptic Transmission at Drosophila larval Neuromuscular Junction

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disease that develops in some premutation (PM) carriers of the FMR1 gene with alleles bearing 55-200 CGG repeats. The discovery of a broad spectrum of clinical and cell developmental abnormalities among PM carriers with or without FXTAS and in model systems suggests that neurodegeneration seen in FXTAS could be the inevitable end-result of pathophysiological processes set during early development. Hence, it is imperative to trace early PM-induced pathological abnormalities. Previous studies have shown that transgenic Drosophila carrying PM-length CGG repeats are sufficient to cause neurodegeneration. Here, we used the same transgenic model to understand the effect of CGG repeats on the structure and function of the developing nervous system. We show that presynaptic expression of CGG repeats restricts synaptic growth, reduces the number of synaptic boutons, leads to aberrant presynaptic varicosities, and impairs synaptic transmission at the larval neuromuscular junctions. The postsynaptic analysis shows that both glutamate receptors and subsynaptic reticulum proteins were normal. However, a high percentage of boutons show a reduced density of Bruchpilot protein, a key component of presynaptic active zones required for vesicle release. The electrophysiological analysis shows a significant reduction in quantal content, a measure of total synaptic vesicles released per excitation potential. Together, these findings suggest that synapse perturbation caused by rCGG repeats mediates presynaptically during larval NMJ development. We also suggest that the stress-activated c-Jun N-terminal kinase protein Basket and CIDE-N protein Drep-2 positively mediate Bruchpilot active zone defects caused by rCGG repeats.

Parkinsonism Versus Concomitant Parkinson's Disease in Fragile X-Associated Tremor/Ataxia Syndrome

BACKGROUND

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder associated with premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. FXTAS is characterized by the presence of ubiquitin-positive inclusions in neurons and astrocytes and by cerebellar tremor and ataxia. Parkinsonism has been reported in FXTAS, but most patients lack the characteristic rest tremor and severe rigidity seen in idiopathic Parkinson's disease (PD).

OBJECTIVE

To describe the frequency of concomitant PD in FXTAS.

METHODS

We reviewed the medical record of 40 deceased patients diagnosed with FXTAS and performed a pathology analysis to confirm both FXTAS and PD.

RESULTS

Clinical histories indicated that 5 FXTAS patients were diagnosed with idiopathic PD and 2 with atypical parkinsonian syndrome. After pathological examination, we found that 7 patients in the PD clinical diagnosis group had dopaminergic neuronal loss; however, only 2 of 7 presented Lewy bodies (LBs) in the substantia nigra. Therefore, a total of 5% of the 40 cohort patients met the pathologic criteria for the concomitant diagnosis of FXTAS and PD. In addition, 2 patients not clinically diagnosed with PD also had nigral neuronal loss with LBs in substantia nigra. In total 10% of these 40 patients had LBs.

CONCLUSION

This report expands our understanding of clinical symptoms and unusual presentations in patients with FXTAS and the concept that the parkinsonism found in FXTAS is sometimes indistinguishable from PD. We propose that FMR1 should be recognized as one of the exceptional genetic causes of parkinsonism with presynaptic dopaminergic loss and LBs.

ATP11B deficiency leads to impairment of hippocampal synaptic plasticity

Synaptic plasticity is known to regulate and support signal transduction between neurons, while synaptic dysfunction contributes to multiple neurological and other brain disorders; however, the specific mechanism underlying this process remains unclear. In the present study, abnormal neural and dendritic morphology was observed in the hippocampus following knockout of Atp11b both in vitro and in vivo. Moreover, ATP11B modified synaptic ultrastructure and promoted spine remodeling via the asymmetrical distribution of phosphatidylserine and enhancement of glutamate release, glutamate receptor expression, and intracellular Ca2+ concentration. Furthermore, experimental results also indicate that ATP11B regulated synaptic plasticity in hippocampal neurons through the MAPK14 signaling pathway. In conclusion, our data shed light on the possible mechanisms underlying the regulation of synaptic plasticity and lay the foundation for the exploration of proteins involved in signal transduction during this process.

Repeat-associated non-AUG (RAN) translation: insights from pathology

More than 40 different neurological diseases are caused by microsatellite repeat expansions. Since the discovery of repeat-associated non-AUG (RAN) translation by Zu et al. in 2011, nine expansion disorders have been identified as RAN-positive diseases. RAN proteins are translated from different types of nucleotide repeat expansions and can be produced from both sense and antisense transcripts. In some diseases, RAN proteins have been shown to accumulate in affected brain regions. Here we review the pathological and molecular aspects associated with RAN protein accumulation for each particular disorder, the correlation between disease pathology and the available in vivo models and the common aspects shared by some of the newly discovered RAN proteins.

Cerebellar Mild Iron Accumulation in a Subset of FMR1 Premutation Carriers with FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder associated with premutation alleles of the FMR1 gene. Iron is essential for many facets of cell metabolism in the brain but when altered is likely to contribute to the development of neurodegenerative diseases. We previously reported that iron accumulates in the choroid plexus and the putamen in FXTAS and that the level and distribution of key iron-binding proteins are also altered, suggesting a potential alteration of iron metabolism in the brain. Here, we hypothesize that iron metabolism is also altered in the FXTAS cerebellum. To test this hypothesis, we used cerebellum samples collected from FXTAS and control subjects and measured the amount of iron contained within the cerebellar cortex and dentate nucleus. We found that the number of iron deposits increased in the cerebellum only in a subset of cases of FXTAS. This accumulation is likely to be mediated by factors other than or in addition to CGG-repeat coupled pathology. Thus, iron deposition in the cerebellum cannot be used as a hallmark of FXTAS pathogenesis.

Fragile X-Associated Tremor/Ataxia Syndrome: From Molecular Pathogenesis to Development of Therapeutics

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder caused by a premutation CGG repeat expansion (55-200 repeats) within the 5' UTR of the fragile X gene (FMR1). FXTAS is characterized by intension tremor, cerebellar ataxia, progressive neurodegeneration, parkinsonism and cognitive decline. The development of transgenic mouse and Drosophila melanogaster models carrying an expanded CGG repeat has yielded valuable insight into the pathophysiology of FXTAS. To date, we know of two main molecular mechanisms of this disorder: (1) a toxic gain of function of the expanded CGG-repeat FMR1 mRNA, which results in the binding/sequestration of the CGG-binding proteins; and (2) CGG repeat-associated non-AUG-initiated (RAN) translation, which generates a polyglycine peptide toxic to cells. Besides these CGG-mediated mechanisms, recent studies have shed light on additional mechanisms of pathogenesis, such as the antisense transcript ASFMR1, mitochondrial dysfunction, DNA damage from R-loop formation and 5-hydroxymethylcytosine (5hmC)-mediated epigenetic modulation. Here we summarize the recent progress towards understanding the etiology of FXTAS and provide an overview of potential treatment strategies.

FMR1 premutation with Prader-Willi phenotype and fragile X-associated tremor/ataxia syndrome

This is a report of FMR1 premutation with Prader-Willi phenotype (PWP) and FXTAS. Although the PWP is common in fragile X syndrome (FXS), it has never been described in someone with the premutation. The patient presented intranuclear inclusions, severe obesity, hyperphagia, and ADHD symptoms, typical of the PWP in FXS. In addition, the autopsy revealed multiple architectural cortical abnormalities.

Abnormal trajectories in cerebellum and brainstem volumes in carriers of the fragile X premutation

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder typically affecting male premutation carriers with 55-200 CGG trinucleotide repeat expansions in the FMR1 gene after age 50. The aim of this study was to examine whether cerebellar and brainstem changes emerge during development or aging in late life. We retrospectively analyzed magnetic resonance imaging scans from 322 males (age 8-81 years). Volume changes in the cerebellum and brainstem were contrasted with those in the ventricles and whole brain. Compared to the controls, premutation carriers without FXTAS showed significantly accelerated volume decrease in the cerebellum and whole brain, flatter inverted U-shaped trajectory of the brainstem, and larger ventricles. Compared to both older controls and premutation carriers without FXTAS, carriers with FXTAS exhibited significant volume decrease in the cerebellum and whole brain and accelerated volume decrease in the brainstem. We therefore conclude that cerebellar and brainstem volumes were likely affected during both development and progression of neurodegeneration in premutation carriers, suggesting that interventions may need to start early in adulthood to be most effective.

Fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that affects some but not all carriers of small, noncoding CGG-repeat expansions (55-200 repeats; premutation) within the fragile X gene (FMR1). Principal features of FXTAS include intention tremor, cerebellar ataxia, Parkinsonism, memory and executive function deficits, autonomic dysfunction, brain atrophy with white matter disease, and cognitive decline. Although FXTAS was originally considered to be confined to the premutation range, rare individuals with a gray zone (45-54 repeats) or an unmethylated full mutation (>200 repeats) allele have now been described, the constant feature of the disorder remaining the requirement for FMR1 expression, in contradistinction to the gene silencing mechanism of fragile X syndrome. Although transcriptional activity is required for FXTAS pathogenesis, the specific trigger(s) for FXTAS pathogenesis remains elusive, highlighting the need for more research in this area. This need is underscored by recent neuroimaging findings of changes in the central nervous system that consistently appear well before the onset of clinical symptoms, thus creating an opportunity to delay or prevent the appearance of FXTAS.

Phenotypes of hypofrontality in older female fragile X premutation carriers

OBJECTIVE

To investigate the nature of cognitive impairments and underlying brain mechanisms in older female fragile X premutation carriers with and without fragile X-associated tremor/ataxia syndrome (FXTAS).

METHODS

Extensive neuropsychological testing and cognitive event-related brain potentials (ERPs; particularly, the auditory P300) were examined in 84 female participants: 33 fragile X premutation carriers with FXTAS (mean age = 62.8 years), 25 premutation carriers without FXTAS (mean age = 55.4 years), and 26 normal healthy controls (mean age = 59.3 years).

RESULTS

Both premutation groups exhibited executive dysfunction on the Behavioral Dyscontrol Scale, with subtle impairments in inhibition and performance monitoring in female carriers without FXTAS, and more substantial deficits in FXTAS women. However, the female carrier group without FXTAS showed more pronounced deficiencies in working memory. Abnormal ERPs were recorded over the frontal lobes, where FXTAS patients showed both P300 amplitude reduction and latency prolongation, whereas only decreased frontal P300 amplitudes were found in carriers without FXTAS. These frontal P300 measures correlated with executive function and information processing speed.

INTERPRETATION

The neuropsychological testing and ERP results of the present study provide support for the hypothesis that executive dysfunction is the primary cognitive impairment among older female premutation carriers both with and without FXTAS, although these deficits are relatively mild compared to those in FXTAS males. These findings are consistent with a synergistic effect of the premutation and aging on cognitive impairment among older female fragile X premutation carriers, even in those without FXTAS symptoms.

The cognitive neuropsychological phenotype of carriers of the FMR1 premutation

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder affecting a subset of carriers of the FMR1 (fragile X mental retardation 1) premutation. Penetrance and expression appear to be significantly higher in males than females. Although the most obvious aspect of the phenotype is the movement disorder that gives FXTAS its name, the disorder is also accompanied by progressive cognitive impairment. In this review, we address the cognitive neuropsychological and neurophysiological phenotype for males and females with FXTAS, and for male and female unaffected carriers. Despite differences in penetrance and expression, the cognitive features of the disorder appear similar for both genders, with impairment of executive functioning, working memory, and information processing the most prominent. Deficits in these functional systems may be largely responsible for impairment on other measures, including tests of general intelligence and declarative learning. FXTAS is to a large extent a white matter disease, and the cognitive phenotypes observed are consistent with what some have described as white matter dementia, in contrast to the impaired cortical functioning more characteristic of Alzheimer's disease and related disorders. Although some degree of impaired executive functioning appears to be ubiquitous among persons with FXTAS, the data suggest that only a subset of unaffected carriers of the premutation - both female and male - demonstrate such deficits, which typically are mild. The best-studied phenotype is that of males with FXTAS. The manifestations of cognitive impairment among asymptomatic male carriers, and among women with and without FXTAS, are less well understood, but have come under increased scrutiny.

Visual motion processing deficits in infants with the fragile X premutation

Background

Fragile X syndrome (FXS) results from a trinucleotide repeat expansion (full mutation >200 cytosine-guanine-guanine (CGG) repeats) in the FMR1 gene, leading to a reduction or absence of the gene’s protein product, fragile X mental retardation protein (FMRP), ultimately causing cognitive and behavioral impairments that are characteristic of the syndrome. In our previous work with infants and toddlers with FXS, we have been able to describe much about their cognitive and visual processing abilities. In light of recent work on the mild cognitive deficits and functional and structural brain differences that are present in adults with the fragile X (FX) premutation, in the present study we examined whether some of the low-level visual processing deficits we have observed in infants with FXS would also be present in infants with the FX premutation (55–200 CGG repeats).

Methods

We chose a contrast detection task using second-order motion stimuli on which infants with FXS previously showed significantly increased detection thresholds (Vision Res 48:1471–1478, 2008). Critically, we also included a developmental delay comparison group of infants with Down syndrome (DS), who were matched to infants with FXS on both chronological and mental age, to speak to the question of whether this second-order motion processing deficit is a FX-specific phenomenon.

Results

As reported previously, infants with the FX full mutation showed motion contrast detection threshold levels that were significantly higher than age-matched typically developing control infants. Strikingly, the motion detection contrast levels of FX premutation infants were also significantly higher than typically developing (TD) infants and not significantly different from the group of infants with FXS or with DS.

Conclusions

These results, which are in keeping with a growing body of evidence on the mild cognitive and perceptual processing deficits and functional and structural brain differences that are present in adults and older children with the FX premutation, underscore the pressing need to study and describe the processing capabilities of infants and toddlers with the FX premutation.

Fragile X-associated tremor/ataxia syndrome: influence of the FMR1 gene on motor fiber tracts in males with normal and premutation alleles

IMPORTANCE

Individuals with the fragile X premutation express expanded CGG repeats (repeats 55-200) in the FMR1 gene and elevated FMR1 messenger RNA (mRNA) levels, both of which may underlie the occurrence of the late-onset neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Because the core feature of FXTAS is motor impairment, determining the influence of FMR1 mRNA levels on structural connectivity of motor fiber tracts is critical for a better understanding of the pathologic features of FXTAS.

OBJECTIVE

To examine the associations of CGG repeat and FMR1 mRNA with motor-related fiber tracts in males with premutation alleles.

DESIGN AND SETTING

A case-control study conducted at the University of California, Davis, from April 1, 2008, through August 31, 2009. All data were collected masked to the carrier status of the FMR1 gene.

PARTICIPANTS

Thirty-six male premutation carriers with FXTAS and 26 male premutation carriers without FXTAS were recruited through their family relationships with children affected by fragile X syndrome. The controls were 34 unaffected family members and healthy volunteers from the local community.

MAIN OUTCOMES AND MEASURES

The CGG repeat lengths and FMR1 mRNA expression levels in peripheral blood lymphocytes, motor functioning, and white matter structural integrity that were estimated using diffusion tensor imaging. After data collection, we selected 4 motor tracts to reconstruct using diffusion tensor tractography, namely, the middle and superior cerebellar peduncles, descending motor tracts (containing the corticospinal, corticobulbar, and corticopontine tracts), and the anterior body of the corpus callosum.

RESULTS

All fiber tracts exhibited weaker structural connectivity in the FXTAS group (decreased 5%-53% from controls, P ≤ .02). Genetic imaging correlation analysis revealed negative associations of CGG repeat length and FMR1 mRNA with connectivity strength of the superior cerebellar peduncles in both premutation groups (partial r² = 0.23-0.33, P ≤ .004). In addition, the measurements from the corpus callosum and superior cerebellar peduncles revealed a high correlation with motor functioning in all 3 groups (r between partial least square predicted and actual test scores = 0.41-0.56, P ≤ .04).

CONCLUSIONS AND RELEVANCE

Distinct pathophysiologic processes may underlie the structural impairment of the motor tracts in FXTAS. Although both the corpus callosum and superior cerebellar peduncles were of great importance to motor functioning, only the superior cerebellar peduncles exhibited an association with the elevated RNA levels in the blood of fragile X premutation carriers.

Advances in clinical and molecular understanding of the FMR1 premutation and fragile X-associated tremor/ataxia syndrome

Fragile X syndrome, the most common heritable form of cognitive impairment, is caused by epigenetic silencing of the fragile X (FMR1) gene owing to large expansions (>200 repeats) of a non-coding CGG-repeat element. Smaller, so-called premutation expansions (55-200 repeats) can cause a family of neurodevelopmental phenotypes (attention deficit hyperactivity disorder, autism spectrum disorder, seizure disorder) and neurodegenerative (fragile X-associated tremor/ataxia syndrome [FXTAS]) phenotypes through an entirely distinct molecular mechanism involving increased FMR1 mRNA production and toxicity. Results of basic cellular, animal, and human studies have helped to elucidate the underlying RNA toxicity mechanism, while clinical research is providing a more nuanced picture of the range of clinical manifestations. Advances of knowledge on both mechanistic and clinical fronts are driving new approaches to targeted treatment, but two important necessities are emerging: to define the extent to which the mechanisms contributing to FXTAS also contribute to other neurodegenerative and medical disorders, and to redefine FXTAS in view of its differing presentations and associated features.

Fragile X-associated tremor/ataxia syndrome (FXTAS): pathology and mechanisms

Since its discovery in 2001, our understanding of fragile X-associated tremor/ataxia syndrome (FXTAS) has undergone a remarkable transformation. Initially characterized rather narrowly as an adult-onset movement disorder, the definition of FXTAS is broadening; moreover, the disorder is now recognized as only one facet of a much broader clinical pleiotropy among children and adults who carry premutation alleles of the FMR1 gene. Furthermore, the intranuclear inclusions of FXTAS, once thought to be a CNS-specific marker of the disorder, are now known to be widely distributed in multiple non-CNS tissues; this observation fundamentally changes our concept of the disease, and may provide the basis for understanding the diverse medical problems associated with the premutation. Recent work on the pathogenic mechanisms underlying FXTAS indicates that the origins of the late-onset neurodegenerative disorder actually lie in early development, raising the likelihood that all forms of clinical involvement among premutation carriers have a common underlying mechanistic basis. There has also been great progress in our understanding of the triggering event(s) in FXTAS pathogenesis, which is now thought to involve sequestration of one or more nuclear proteins involved with microRNA biogenesis. Moreover, there is mounting evidence that mitochondrial dysregulation contributes to the decreased cell function and loss of viability, evident in mice even during the neonatal period. Taken together, these recent findings offer hope for early interventions for FXTAS, well before the onset of overt disease, and for the treatment of other forms of clinical involvement among premutation carriers.

Intranuclear inclusions in a fragile X mosaic male

Lack of the fragile X mental retardation protein leads to Fragile X syndrome (FXS) while increased levels of FMR1 mRNA, as those observed in premutation carriers can lead to Fragile X- associated tremor ataxia syndrome (FXTAS). Until recently, FXTAS had been observed only in carriers of an FMR1 premutation (55–200 CGG repeats); however the disorder has now been described in individuals carriers of an intermediate allele (45–54 CGG repeats) as well as in a subject with a full mutation with mosaicism.

Here, we report on molecular and clinical data of a male FMR1 mosaic individual with full and premutation alleles. Molecular analysis of FMR1 and FMRP expression in this subject is consistent with a FXS phenotype. We observed reduced expression of FMRP in both peripheral blood and brain leading to the FXS diagnosis. In addition, a dramatic 90% depletion of both FMR1 mRNA and FMRP levels was observed in the blood, as normally observed in FXS cases, and an even greater depletion in the brain. A clinical report of this patient, at age 71, described neurodegenerative signs of parkinsonism that were likely, in retrospect, part of a FXTAS scenario as post-mortem examination shows the presence of intranuclear inclusions, the hallmark pathology of FXTAS.

The findings presented in this study indicate co-morbidity for both FXS and FXTAS in this individual carrying both full and premutation FMR1 alleles. In addition, based on symptoms and pathological and molecular evidence, this report suggests the need to redefine the diagnostic criteria of FXTAS.

Impaired activity-dependent FMRP translation and enhanced mGluR-dependent LTD in Fragile X premutation mice

Fragile X premutation-associated disorders, including Fragile X-associated Tremor Ataxia Syndrome, result from unmethylated CGG repeat expansions in the 5' untranslated region (UTR) of the FMR1 gene. Premutation-sized repeats increase FMR1 transcription but impair rapid translation of the Fragile X mental retardation protein (FMRP), which is absent in Fragile X Syndrome (FXS). Normally, FMRP binds to RNA and regulates metabotropic glutamate receptor (mGluR)-mediated synaptic translation, allowing for dendritic synthesis of several proteins. FMRP itself is also synthesized at synapses in response to mGluR activation. However, the role of activity-dependent translation of FMRP in synaptic plasticity and Fragile X-premutation-associated disorders is unknown. To investigate this question, we utilized a CGG knock-in mouse model of the Fragile X premutation with 120-150 CGG repeats in the mouse Fmr1 5' UTR. These mice exhibit increased Fmr1 mRNA production but impaired FMRP translational efficiency, leading to a modest reduction in basal FMRP expression. Cultured hippocampal neurons and synaptoneurosomes derived from CGG KI mice demonstrate impaired FMRP translation in response to the group I mGluR agonist 3,5-dihydroxyphenylglycine. Electrophysiological analysis reveals enhanced mGluR-mediated long-term depression (mGluR-LTD) at CA3-CA1 synapses in acute hippocampal slices prepared from CGG KI mice relative to wild-type littermates, similar to Fmr1 knockout mice. However, unlike mGluR-LTD in mice completely lacking FMRP, mGluR-LTD in CGG knock-in mice remains dependent on new protein synthesis. These studies demonstrate partially overlapping synaptic plasticity phenotypes in mouse models of FXS and Fragile X premutation disorders and support a role for activity-dependent synthesis of FMRP in enduring forms of synaptic plasticity.

Influence of the fragile X mental retardation (FMR1) gene on the brain and working memory in men with normal FMR1 alleles

The fragile X mental retardation 1 (FMR1) gene plays an important role in the development and maintenance of neuronal circuits that are essential for cognitive functioning. We explored the functional linkage(s) among lymphocytic FMR1 gene expression, brain structure, and working memory in healthy adult males. We acquired T1-weighted and diffusion tensor imaging from 37 males (18-80 years, mean ± SD= 40.7 ± 17.3 years) with normal FMR1 alleles and performed genetic and working memory assessments. Brain measurements were obtained from fiber tracts important for working memory (i.e. the arcuate fasciculus, anterior cingulum bundle, inferior longitudinal fasciculus, and the genu and anterior body of the corpus callosum), individual voxels, and whole brain. Both FMR1 mRNA and protein (FMRP) levels exhibited significant associations with brain measurements, with FMRP correlating positively with gray matter volume and white matter structural organization, and FMR1 mRNA negatively with white matter structural organization. The correlation was widespread, impacting rostral white matter and 2 working-memory fiber tracts for FMRP, and all cerebral white matter areas except the fornix and cerebellar peduncles and all 4 fiber tracts for FMR1 mRNA. In addition, the levels of FMR1 mRNA as well as the fiber tracts demonstrated a significant correlation with working memory performance. While FMR1 mRNA exhibited a negative correlation with working memory, fiber tract structural organization showed a positive correlation. These findings suggest that the FMR1 gene is a genetic factor common for both working memory and brain structure, and has implications for our understanding of the transmission of intelligence and brain structure.